Critical dimension control is important in integrated circuit manufacturing processes, particularly with respect to gate critical dimensions, which impact transistor performance. Controlling critical dimensions is becoming increasingly important as dimensions of gate elements, such as polysilicon gate elements, fall within the deep sub-micron range. Gate dimensions generally need to be controlled to within 2-3% of the target dimension. In 90 nm devices, the run-to-run gate critical dimension variation is required to be controlled to within 2-3 nm. For anticipated 65 nm devices, acceptable gate critical dimension variation is even smaller. Therefore, accurate monitoring of critical dimensions is of increasing importance in the industry.
Presently, critical dimensions are determined by actually measuring line widths of sampled wafers outside of the process that determines the critical dimension. Measurements on a sample wafer are typically taken by a separate scanning electron beam (SEM) metrology tool. A wafer is removed from the manufacturing process and its critical dimensions are then measured using the tool. The wafer can then be returned to the manufacturing process. This method, however, is slow, requires additional expensive hardware, requires significant additional processing steps and can cause so called “e-beam” damage to layers formed on the wafer. Further, the accuracy of these SEM techniques may not be sufficient for newer generations of integrated circuits (e.g., 65-90 nm devices).
In some processes, a separate optical tool is used to measure critical dimension. However, this methodology is also performed by a separate tool after etch processing and requires formation of grating structures on the wafer, oblique angle light sources, high clean surfaces and the use of complicated mathematical models to determine critical dimension.
Therefore, there is a need for a non-destructive method of in-situ measuring critical dimensions of features of integrated circuits. Still further, there remains a need to provide for in-situ measurement of critical dimensions without costly hardware additions or upgrades.